How to keep parts of single object easily selectable in 2.8?
You want vertex groups as you've mentioned.

So go into Edit Mode, then press 1 to make sure you are selecting vertices.

Go into plan view with NumPad 7 so we're all setup.

Now click open the Mesh Properties Panel and expand Vertex Groups if not already visible.

Now the essential part. Click the + button to create a new vertext group.

Double click it to rename Hand



Then in 3D view box select all the vertices of your hand and click Assign.

This assigns the selected vertices to the group.

Now if you deselect all vertices, you can select them again by clicking Select.

You should now have all vartices which were assigned to the group selected again.